This invention relates to a process for forming an allophanate-modified polymethylene poly(phenylisocyanate) comprising the steps of reacting a small amount of epoxide, along with alcohol, to polymethylene poly(phenylisocyanate) (PMDI) to form an intermediate, which is converted to allophanate upon addition of a metal divalent catalyst. This invention also relates to a process for forming an allophanate-modified PMDI comprising the steps of adding a small amount of epoxide, to PMDI to form an intermediate, which is converted to allophanate upon addition of an alcohol and a metal divalent catalyst. The present invention also relates to these allophanate-modified polymethylene poly(phenyl isocyanates).
Allophanate-modified methylene bis(phenyisocyanate) (MDI) have seen increased use in many applications ranging from polyurethane foams to coatings. Polymeric MDI, however, is problematic toward allophanate formation due to its high acidity. The high acidity neutralizes the allophanate catalyst and thus inhibits allophanate formation. Extremely high catalyst levels, high reaction temperatures, and long reaction times are required to form allophanates using PMDI. It was therefore, an object of the present invention to develop a process for preparing PMDI-based allophanates using mild conditions and low catalyst levels.
The process for the preparation of allophanate-modified polymeric aromatic isocyanates is known. For example, U.S. Pat. No. 5,902,840 discloses a method for the preparation of allophanate modified PMDI. The disclosed process comprises the step of reacting PMDI with a monofunctional aliphatic alcohol to form an intermediate, and reacting this intermediate at elevated temperature in the presence of a divalent metal catalyst. The modified polyisocyanates, along with a phenolic resole resin are added to a foundry aggregate to form a foundry mix which is shaped and cured with a gaseous amine curing catalyst by the cold-box process.
U.S. Pat. No. 5,342,881 discloses a stabilized organic poly-isocyanate composition comprising an organic polyisocyanate, one or more compounds having at least one epoxide group of a specific formula, and a stabilizing silylating agent and/or alkylating agent. The polyisocyanate/epoxide mixture is optionally heated before the stabilizing agents are added. These polyisocyanates are useful in the production of polyisocyanate addition products. The silylating agent or alkylating agent is necessary to form a stable polyisocyanate component.
U.S. Pat. No. 5,783,652 relates to mixtures of urethane prepolymers of allophanate-modified diphenylmethane diisocyanates with epoxides. The addition of the epoxide increases the reactivity of these prepolymers. Polyurethanes are formed from these mixtures based on prepolymers of diphenylmethane diisocyanates wherein less catalyst is necessary due to the increased reactivity of the prepolymer.
U.S. Pat. No. 5,258,417 describes storage stable blends of polymethylene polyphenylpolyisocyanates having viscosities from 15 to 37 cps that are prepared by adding diphenylmethane diisocyanates to the polymeric MDI in amounts such that the two-ring isomer content does not exceed 80 percent by weight of the blend. These blends within the limitations given are stable for up to six months at 25xc2x0 C. The ring distribution of the blend comprises: 1) from 60 percent to about 75 percent by weight 4,4xe2x80x2-diphenylmethane diisocyanate, 2) from 4 percent to about percent by weight 2,4-diphenylmethane diisocyanate, 3) less than 1 percent by weight 2,2xe2x80x2-diphenylmethane diisocyanate, 4) from 8 percent to about 15 percent by weight of a 3-ring polymethylene polyphenylpolyisocyanate, and, 5) from 5 percent to about 15 percent by weight of a 4-ring and higher oligomeric polymethylene polyphenylpolyisocyanate.
Several patents disclose that the acidity of PMDI can be reduced, and/or the reactivity of PMDI can be improved or increased by the addition of epoxides to the reaction mixture. These patents include for example, U.S. Pat. Nos. 3,793,362; 3,925,437; 5,726,240; and CA 915,846.
U.S. Pat. No. 5,726,240 discloses that the process of producing polyurethanes by reacting a polyisocyanate component with an isocyanate-reactive component typically occurs in the presence of one or more catalysts. Suitable catalysts include, for example, organometallic catalysts and tertiary amine compounds. The quantity of catalyst necessary is dependent on the acidity of the particular polyisocyanate component used in this process. As the acidity of the polyisocyanate increases, the reactivity of the polyisocyanate decreases. In other words, it takes longer for the reaction to occur unless additional catalyst is added. In order for the reaction to proceed within a specific time period consistently, it is usually necessary to increase or decrease the quantity of catalyst used to achieve constant reactivity profiles using blends of polyisocyanates with various acidity levels.
Additionally, U.S. Pat. No. 5,726,240 discloses that with the use of mixtures of polymethylene polyisocyanate blends with epoxides in the process of producing polyurethanes, it is possible to reduce the quantity of catalyst. The reactivity profiles of catalyzed formulations containing these isocyanate blends are significantly more stable, regardless of the acidity level of the polyisocyanate.
It is surprising that the allophanate modified PMDI prepared according to the present invention is stable as measured by viscosity over time. This is surprising because this stability is in direct conflict with prior disclosures. For instance, U.S. Pat. No. 5,342,881 deals with the stabilization of PMDI/epoxide mixtures by the addition of silylated acids and/or alkylated acids. Table 1 of this reference shows that, unless a stabilizer is added to the system, large increases in viscosity are seen. Furthermore, Column 7, lines 34-45 imply that any modification of PMDI, either urethane or allophanate based, must also be stabilized when epoxides are added to them.
This instability is further illustrated in U.S. Pat. No. 5,783,652, which describes the process of adding epoxides to urethane prepolymers of an allophanate-modified diphenylmethane diisocyanate. Tables A and B of ""652 illustrate that the addition of epoxides to MDI allophanates gives unstable products. In column 2, lines 16-18, U.S. Pat. No. 5,783,652 specifically states that xe2x80x9cthe treated allophanate-modified MDI is not stable relative to the NCO content and viscosity at normal storage temperatures of 25 to 50xc2x0 C.xe2x80x9d
The present invention provides storage stable allophanate modified PMDI products at normal storage temperatures. For example, an allophanate modified PMDI was prepared from a PMDI component, isobutanol, epoxide, allophanate catalyst. After 14 days of storage at 25xc2x0 C., a decrease in viscosity was observed in this sample. According to U.S. Pat. No. 5,783,652, a viscosity increase should have occurred at this temperature indicating that the product was unstable. This decrease in viscosity is contrary to what is disclosed or suggested by U.S. Pat. No. 5,783,652.
Table 1 from U.S. Pat. No. 5,342,881 illustrates that a decrease in % NCO is accompanied by an increase in viscosity. Furthermore, the larger the % NCO decrease, the greater the viscosity increase. The combination of U.S. Pat. Nos. 5,342,881 and 5,783,652 would lead one skilled in the art to conclude that 1) addition of epoxides to allophanate modified isocyanates results in unstable products that increase in viscosity, and 2) addition of some type of stabilizer (as in U.S. Pat. Nos. 5,342,881 and 5,783,652) is necessary to prevent this instability.
The present invention thus provides an improved process over U.S. Pat. No. 5,902,840 for preparing allophanate modified PMDI products. PMDI plus only epoxides yield unstable products as disclosed by U.S. Pat. No. 5,342,881. The formation of urethane does not stabilize PMDI/epoxide mixture. U.S. Pat. No. 5,726,240 shows that epoxides destabilize allophanate modified monomeric MDI. It is evident from the examples of U.S. Pat. No. 5,726,240 that the viscosity of the allophanate modified MDI increases significantly over a time period of about 50 minutes to 100,000 mPa.s. In the present invention, in-situ formation of allophanates in a PMDI/epoxide mixture gives a more stable product. This is unexpected.
This invention relates to a process for forming allophanate-modified PMDI comprising the steps of reacting a small amount of epoxide, along with alcohol, to polymethylene poly(phenylisocyanate) (PMDI) to form an intermediate, which is converted to allophanate upon addition of a metal divalent catalyst. This invention also relates to a process for forming an allophanate-modified polymethylene poly(phenylisocyanate) comprising the steps of adding a small amount of epoxide, to polymethylene poly(phenylisocyanate) (PMDI) to form an intermediate, which is converted to allophanate upon addition of an alcohol and a metal divalent catalyst. The present process for preparing PMDI-based allophanates uses mild conditions and low catalyst levels to yield viscosity stable allophanate-modified PMDI. The present invention also relates to novel allophanate-modified polymethylene poly(phenyl isocyanates).